Fig. 12.18 are approximately equal). As indicated earlier, the additional oxygen used during recovery from exercise represents the energy requirements for restoring normal cellular metabolite levels.
The ability of muscle to meet energy needs is a major determinant of the duration of the exercise. However, fatigue is not the result of depletion of energy stores. Instead, metabolic byproducts seem to be important factors in the onset of fatigue. Fatigue may potentially occur at any of the points involved in muscle contraction, from the brain to the muscle cells, as well as in the cardiovascular and respiratory systems that maintain energy supplies (i.e., fatty acids and glucose) and oxygen delivery to the exercising muscle.
Several factors have been implicated in muscle fatigue. During brief periods of tetany, the oxygen supply to the muscle is adequate as long as the circulation is intact. However, the force/stress generated during these brief
Rate of energy expenditure •Fig. 12.18. An Oxygen Debt Is Incurred by the Exercising of Muscle When the Rate of Energy Expenditure Exceeds the Rate of Energy Production by Oxidative Metabolism. Upper panel, Energyexpenditureduringstrenuousexercise.Lower panel, Energyexpenditureduringenduranceexercise.Seetextfordetails.
tetanic periods decays rapidly to a level that can be maintained for long periods (
Fig. 12.19 ). This decay represents the rapid and almost total failure of the fast motor units. The decline in force/stress is paralleled by depletion of glycogen and creatine phosphate stores and the accumulation of lactic acid. Of importance is that the decline in force/ stress occurs when the ATP pool is not greatly reduced, so that the muscle fibers do not go into rigor. In contrast, the slow motor units are able to meet the energy demands of fibers under this condition, and they do not exhibit significant fatigue, even after many hours. Evidently, some factor associated with energy metabolism can inhibit contraction (e.g., in the fast fibers), but this factor has not been clearly identified.